1. Field of Invention
This invention pertains to underdrain lateral systems, such as those of the type utilized in a gravity filter bed.
2. Related Art and other Considerations
U.S. Pat. No. 5,156,738 to Maxson, incorporated herein by reference, illustrates an underdrain lateral system of the type having a curved distributor member or screen which is situated above a fiat baseplate. As illustrated in the Maxson patent and is understood by those skilled in the art, the underdrain lateral system is positioned below a media bed, such that liquid, usually water, exiting overflow troughs above the media bed pass through the media. During normal operation, water which has passed through the media bed falls into the underdrain lateral system, and particularly into upper concave surfaces of the screen. The upper concave surface of the screen has perforations such as thin slots which are sized so as to retain particles which make up the media bed. Water passing through the screen is subsequently discharged from the underdrain lateral system. As opposed to normal operation, a backwash mode is utilized to clean the media. In the backwash mode, water and air are directed upwardly through the screen perforations and media bed in a uniform manner so that the material previously filtered out and captured by the media bed will be picked up and carried upwardly where it will be discharged through the overflow trough.
The underdrain lateral system shown in the Maxson patent is formed of stainless steel and has its screen portion situated between two up-turned, side edge portions of its elongated flat baseplate. Its screen portion is formed by helically winding and welding a metal wire to a channel base support as described in Geske U.S. Pat. No. 4,096,911. Screens and baseplates which form lateral member assemblies for underdrain lateral systems can be quite large, often measuring as long as 30 feet, but more typically, are about 20 feet. In the prior art, such lateral member assemblies were formed of metal, and accordingly were not only heavy to transport but quite cumbersome, because of their weight, to assemble with other units at the point of use. Furthermore, where the assemblies were to be used with seawater or brackish water, they were quite subject to corrosion, even when fabricated of stainless steel. Also, since stainless steel is quite expensive, the baseplates of metal units are usually relatively thin, allowing them to bow during use unless a substantial number of holddown bars are provided to maintain them in place and flat against the floor. Since considerable labor is required to mount the holddown bars with bolts to the concrete floor of the filter bed, it can be appreciated that a lateral unit requiring fewer holddown bars would be quite desirable.